Scars result from wound healing, which occurs in three separate phases: inflammation, formation of granulation tissue, and matrix formation. (For a review, see Sahl, W. J., and Clever, H., Internat. J. Derm., 1994, 33: 681-691 (part I) and 763-769 (part II)). During the first phase, damage to endothelial cells, complement, and platelets at the wound site release chemotactic factors that result in the infusion of neutrophils, lymphocytes and macrophages, which aids in the removal of infection and foreign debris. As in all inflammatory processes, there is generation of free radicals, which damages cell membranes and results in formation of oxidized proteins and fats, and cross-linked new collagen, laying a scaffold for the next phase.
At the end of the inflammatory phase, the granulation phase begins with an influx of fibroblasts and endothelial cells to the wound. Other key cells in this phase are macrophages and platelets. Macrophages induce the beginning of granulation by releasing platelet-derived growth factor (PDGF), tumor necrosis growth factor (TGF)-.alpha., and an epidermal growth factor-like substance. Activated platelets release epidermal growth factor (EGF), PDGF, TGF-.alpha., and TGF-.beta.. Together these play roles in the re-epithelialization process wherein keratinocytes cells migrate in sheaths over a provisional matrix consisting primarily of fibrin, fibronectin, type V collagen, and tenascin, and produce their own fibronectin receptors.
Once re-epithelilization has occurred, keratinocytes resume their normal differentiated form, and matrix formation begins. Matrix formation consists primarily of the construction of derma matrix, which is regulated by fibroblasts. Chemotaxis of fibroblasts results in the production of abundant quantities of hyaluronate, fibronectin, and types I and III collagen. These components comprise the bulk of the provisional extracellular matrix in the early part of this wound repair phase. Hyaluronic acid (HA) creates an open-weave pattern in the collagen/fibronectin scaffold, facilitating fibroblast movement. HA production falls after about the fifth day of wound healing, and levels of chronroitin sulfate in dermatan sulfate increase. Fibronectin deposits in the collagen, and wound contraction begins. Biochemically during the contraction stage, hyaluronidase and proteinase are present, type I collagen synthesis is stimulated, and increased levels of chronroitin sulfate, dermatin sulfate and proteoglycans are observed; together these restructure the matrix. At the end of the healing process, the final scar shows collagen fibers mostly parallel to the epidermis.
Hypertrophic and keloid-type scars result in extension of scar tissue so that a bulky lesion results. A keloid is an exuberant scar that proliferates beyond the original wound. It should be noted that keloids only occur in humans, often causing burning, stinging and itching sensations as well as cosmetic embarrassment. The etiology of unsightly keloid formation is not known. However, in keloids, fibronectin formation continues for years, while fibronectin formation in normal scars disappears within a few days after wound closure. Keloid scars exhibit a high rate of collage synthesis in comparison to normal scars, and a low proportion of cross-linked collagen.
Hypertrophic scars sometimes are difficult to distinguish from keloid scars histologically and biochemically, but unlike keloids, hypertropic scars remain confined to the injury site and often mature and flatten out over time. Both types secrete larger amounts of collagen than normal scars, but typically the hypertrophic type exhibits declining collagen synthesis after about six months. However, hypertrophic scars contain nearly twice as much glycosaminoglycan as normal scars, and this and enhanced synthetic and enzymatic activity result in significant alterations in the matrix which affects the mechanical properties of the scars, including decreased extensibility that makes them feel firm.
Atrophic scars are characterized by a thinning and diminished elasticity of the skin due to a loss of normal skin architecture. An example of an atrophic scar is striae distensae, also known as stretch marks. Striae commonly occur in postpartum women after childbirth and also during times of larger-than-average weight gain and also in association with steroids. Atrophic scars are sometimes also observed after trauma, infection and disease, and may show loss of surface markings and smoothness or dry, fine wrinkles over time.
Formation of scars, especially hypertrophoic and keloid scars, is dependent on systemic growth factors such as interleukins and other cytokines, and their influence on fibronectin and collagen biosynthesis. Cytokines are released and are present in the wound healing process and sometimes are released in the inflammatory stage. Growth factors and other cytokines vary in the inflammatory stage and are released based, among other complex interactions, upon the redox state of the cells. The presence of free radicals in the inflammatory stage plays an important factor in wound healing. Factors that increase the presence of free radicals, such as infection, radiation, and continued trauma, may instigate hypertrophic and keloid scar formation. It is important to note that cytokines have been suggested to regulate nitric oxide synthetase, which controls the formation of nitric oxide, which plays an important role in signal transduction in the cells. It has also been suggested that nitric oxide synthetase activity is aberrant in keloid scars when compared to normal scar tissue (Lim, T. C., et al., Plastic and Reconst. Surgery, 1996, 98:911-912). Hypertrophic and keloid scars also show inflammatory activity that is not seen in mature scars.
Many scar treatments have been suggested, but few are satisfactory. Treatment of keloid or hypertrophic scars have consisted of surgical excision followed in many cases by graft application. Pressure has also been used to cause scar thinning after injury or scarring. For example, pressure bandages placed over scars have resulted in some scar thinning, but a pressure of at least about 25 mm Hg must be maintained constantly for approximately six months in usual situations for any visually observable effect. Ionizing radiation therapy has also been employed. Other treatments include application of silicone pads to the scar tissue surface, sometimes under pressure provided by an elastomeric bandage, application of silicone gel sheets, with or without added vitamin E (Palmieri, B., et al., J. Derm., 1995, 34: 506-509), and topical or intralesional treatment with corticosteroids.
Scars are one of the strongest forces driving the cosmetic industry. It would be desirable to have alternative, preferably new and improved, treatments for scar reduction and remodeling.